The present invention is directed to air treatment systems and more particularly to an efficient method of recovering the beneficial characteristics of an exhaust airstream in a building or structure and providing such beneficial characteristics to the incoming or supply airstream. This technique may be useful either in summer or winter months, however, finds most use during winter months. For example, in winter months the exhaust airstream is going to be warmer than the incoming airstream and can be used to "pre-heat" the incoming airstream by giving up heat thereto.
It has become in recent years a well known and often used broad concept to utilize exhaust heat from some type of furnace or similar apparatus to preheat either directly or indirectly an incoming or supply airstream. Such is shown in U.S. Pat. No. 4,091,547 to Leigh; as well as various other prior art references. In direct situations the exhaust air passes in heat exchange relation to the supply air. In indirect situations a working fluid is utilized in a conduit system between the two air streams. In the hot stream the working fluid picks up heat and in the cooler airstream the working fluid gives up heat. Further, if the working fluid is "two-phase" it vaporizes in the hot airstream and condenses in the cooler airstream. One problem with indirect heat exchange processes that utilize a two-phase working fluid in prior art devices, however, is that either the two airstreams must be side-by-side, as illustrated by U.S. Pat. No. 4,230,173 to Eastman et al, or else the heat exchanger in the cooler airstream (condenser) must be at a higher elevation than the heat exchanger of the warm airstream (vaporizer) so that the condensate will return to the warm air heat exchanger. Obviously in the case of commercial and industrial buildings, it is generally not advantageous to provide the incoming air conduit adjacent the exhaust air conduit as it may be very expensive. Also, it is generally not desirable or economically feasible to place the heat exchanger of the cooler airstream at an elevation above that of the heat exchanger in the warm airstream. This is particularly true in situations where heat exchange is required or desired in both winter and summer operations, because in such situations the two airstreams are opposite in summer and winter. In other words, during winter operation, the condensate will form adjacent the incoming airstream while in summer operation the condensate will form adjacent the exhaust airstream. Therefore, a change in elevation would not allow for year round applicability.
It is further acknowledged by the present applicant that there are some isolated instances where it is known that heat exchangers may be connected by pipe work for the circulation of working fluid or refrigerant between them by pumping the condensed refrigerant from the cooler of the heat exchangers to the warmer heat exchanger. For example, see U.S. Pat. No. 4,091,547 to Leigh. However, this is a one way application utilized in connection with the operation of a grain drying kiln, and the same problems do not exist.
In the present invention, however, heat exchangers are placed in both the incoming and exhaust airstream of a commercial or industrial air treatment system. A pump is placed in the conduit between the two heat exchangers and a two-phase working fluid is utilized in connection therewith. As a result of the combination of the present invention, higher efficiencies may be obtained at lower operating costs. Since the working fluid is pumped between the two heat exchangers, less heat transfer surface is required because of the high heat transfer rate associated with the two-phase working fluid, as opposed to the situation which would be present if water were utilized in the conduits. With the present invention there is no need to either run supply and exhaust airstreams side-by-side or situate the heat exchanger of the hot airstream below that of the cold airstream. In fact the airstreams may be separated by as much as a hundred feet or more.
In general, the present invention is directed to a heat exchange system for use in connection with incoming and exhaust airstreams, which airstreams may be side-by-side, separated by a hundred feet or more, and at different elevations. The system includes at least one heat exchanger in the incoming airstream and at least one heat exchanger in the exhaust airstream, and the two heat exchangers are connected by supply and return conduits which carry a two-phase working fluid back and forth between the two heat exchangers. As an improved setup, a reversible or bi-directional pump in one of the conduits forces or pumps condensed working fluid from the outlet of the heat exchanger in the cooler airstream to the inlet of the heat exchanger in the warm airstream. Since the pump is bi-directional the system economies may be achieved in either winter or summer operation.
In an alternate embodiment, a second identical system with an additional heat exchanger in each airstream and conduits is added to increase the efficiency of the system by approximately an additional 50%. In such a case the two systems are arranged in counterflow arrangement so that the hot exhaust air entering the first coil (heat exchanger) sees a higher evaporating temperature than the air entering the second coil (heat exchanger).
It is therefore an object of the present invention to provide an improved heat exchange system for air treatment systems having an incoming and exhaust airstream, especially in situations where the incoming and exhaust airstream are separated by considerable distance or in which one of the airstreams is at a different elevation than the other.
It is another object of the present invention to provide an improved heat exchange system of the type described which utilizes a two-phase working fluid.
It is yet another object of the present invention to provide an improved heat exchange system of the type described in which the condensed working fluid is pumped from the cooler airstream to the warmer airstream, whereby the setup is applicable in situations where it would not otherwise operate and whereby the efficiency is considerably improved.